Electrical system, such as general-purpose computer systems, typically include power supplies for accepting primary power, such as the 120 v, 60 Hz primary system in use in the United States, and for providing power in other forms as needed by specific systems. Often the systems, such as computers, require power at several different voltages and as direct current (dc) rather than the primary alternating current (ac). In general, it is desirable for a power supply to operate with maximum efficiency so the least amount of power provided is lost in the conversion process.
An efficient power supply will transfer power to its load with a minimum of losses. This saves on power costs by minimizing the amount of power needed to operate the system or equipment.
As described above, a computer system needs power to drive several subsystems, not all of which use the same voltage. For example, most computers use 5 v dc or 3.3 v dc for operating solid-state circuitry, such as a CPU microprocessor, while 12 v dc may be required for motors to drive disk storage systems. Ideally, a computer should only consume the amount of power that is needed to service its necessary functions. In practice, however, some power is wasted in the power supply, and the efficiency is less than 100% Wasted power usually degrades to heat in the power supply components. The waste heat must be dissipated, or the temperature of the components will rise to a temperature sufficient to damage components, or at least to reduce functionality.
A significant source of power loss in a power supply for a computer system is the process for converting the input primary ac line voltage into regulated dc output voltages. Part of the problem is the combination of relatively high voltage and relatively low frequency in the input line (120 v, 60 Hz in the United States).
In a general-purpose computer, the output voltages required are typically at or below 12 V dc., so the higher input line voltage must be lowered significantly. The voltage reduction is typically accomplished through a transformer. Transformers come in a wide variety of sizes, and a transformer's efficiency depends, among other things, on the frequency of the input line voltage, which operates on the transformer's primary winding. In general, for a higher input frequency, a smaller transformer may be used. Furthermore, due to size constraints in a computer system, a small transformer size is desirable. The low frequency of the input (60 Hz), indicating a large transformer, is therefore a problem.
The conventional method to deal with the problem of low input frequency is to increase the frequency of the input line voltage before providing the input to the transformer. The first step in this process typically is to convert the input ac line voltage into an unregulated dc voltage. This process is called rectification and filtering, and is well known. There are many ways known in the art for accomplishing rectification and filtering.
In the conventional case, the unregulated dc voltage is sampled through a switch in order to increase the frequency. That is, the unregulated dc voltage is chopped into a higher frequency. This is also a well known process, and there are many ways of accomplishing it. Finally, the chopped dc voltage waveform is input into the primary winding of a small high-frequency transformer to complete the step-down process.
The voltage step-down process described above for the conventional case requires the input ac line voltage to be first converted to dc then back to ac before actually being stepped down by a transformer. This ac to dc to ac process is common to most computer system power supplies. There are small losses associated With each of the conversion steps, but this step-down stage of the power supply can be designed to be relatively efficient.
The stepped-down, chopped (ac) waveform from the secondary of the transformer must still be converted into a regulated dc voltage for use by the computer system. An often-used, conventional way that has been used to accomplish this regulation stage of the power supply is with regulation of the input. Firstly, the ac waveform from the secondary is rectified by feeding it through diodes, which have a low resistance to current in one direction, and a high resistance in the opposite direction. Secondly, the output from these diodes is filtered to produce a dc voltage. The filtered dc is the output, which is sampled and fed back to control the duty cycle of the switch used to chop the rectified and filtered ac input line voltage. This process regulates the output dc voltage by controlling the amount of energy that is transferred or coupled through the transformer.
In this conventional method, there is a fixed voltage drop across the diodes. If the required output voltage is low enough, such as 5 V, this fixed voltage drop across the diodes, about 0.7 V to 1 V for a standard P-N junction diode, is a significant percentage of the output voltage. This results in power being dissipated across the diodes, which degrades the efficiency of the power supply.
The diode voltage drop can be minimized through by using Schottky diodes, decreasing the voltage drop to about 0.3 V to 0.7 V, but Schottky diodes are significantly more expensive than standard P-N junction diodes, which increases the cost of the power supply. In general, power supplies following this conventional approach exhibit overall efficiency in the 50% to 75% range.
The fact that power dissipated across the diodes is degraded to heat, further compounds the difficulty. The fact of heat production in the diode circuitry requires the diode arrangement be in a discrete package wherein heat may be controlled, and precludes use of integrated circuit technology in the diode package.
Another disadvantage of regulating the input to create a regulated dc output voltage is that only one voltage level can be regulated with precision. This fact is especially significant in a computer system that requires multiple regulated output dc voltages.
One way manufacturers have dealt with the inability to precisely control more than one output is by running separate switching regulators off one tightly regulated dc output voltage level to provide the multiple output voltages. This design is called the power rail concept. Switching regulators are also known as dc to dc converters, and exploit the relationship between the electrical and magnetic fields in order to regulate voltages. These switching regulators are more efficient than comparable-pass transistor voltage regulators, and one common type of switching regulator used by manufacturers is called a buck converter.
The power rail method deals with the multiple output precision difficulty, but still suffers from inefficiency caused by the rectification diodes used to create the power rail. Furthermore, the power rail is normally regulated at a higher voltage than is needed by any supplied subsystem, and the higher voltage serves as a reference voltage for the dc to dc converters. This incurs a cost penalty, as a separate dc to dc converter is needed for each output voltage.
What is clearly needed is a power supply architecture that avoids the losses created by rectification diodes in the regulation stage, and also allows for implementation of multiple, precisely regulated dc output voltages of differing values without using costly power rail. Avoiding the diode losses increases efficiency of the power supply without requiring costly Schottky diodes and allows most of the power supply elements to be implemented as solid-state devices in silicon.